How do I change the location of the "Temp" folder to be a relative path?
that field seems to accept only EXISTING folders, while "//" stands for "current folder", but it is referred to the "current SAVED file".

https://docs.blender.org/manual/en/dev/data_system/files/relative_paths.html says "You cannot enter relative paths into a new untitled blend-file. Save it before linking to external files."

the "temp" path in user preferences probably is needed absolute (and existing) because blender needs it even for itself to, eg, save the quit.blend file, useful to recover the last session (even if not saved), and that file is saved in the "temp" folder. So, blender MUST know well where exactly it is...

I mean this recover function could not work, probably, without being sure of the "temp" folder